1. Technical Field
This invention relates generally to loud speakers used in audio systems. More particularly, this invention relates to a speaker port with a contour that reduces boundary layer separation.
2. Related Art
There are many types of speaker enclosures. Each enclosure type can affect how sound is produced by the speaker. Typically, a driver is mounted flushed within the speaker enclosure. The driver usually has a vibrating diaphragm for emitting sound waves in front of a cone. As the diaphragm moves back and forth, rear waves are created behind the cone as well. Different enclosures types have different ways of handling these “rear” waves.
Many speakers take advantage of these rear waves to supplement forward sound waves produced by the cone. FIGS. 1 and 2 show a bass reflex enclosure that takes advantage of the rear waves. The enclosure has a small port. The backward motion of the diaphragm excites the resonance created by the spring of air inside the speaker enclosure and the mass contained within the port. The length and area of the port are generally sized to tune this resonant frequency. The port and speaker resonance is very efficient so the cone motion is reduced to near zero thereby greatly enhancing the bandwidth and the maximum output of the system that would otherwise be limited by the excursion of the cone.
In many speaker enclosures, sound waves passing through the port generate noise due to boundary layer separation. A sudden expansion or discontinuity in the cross-sectional area of the port can cause boundary layer separation of the sound waves from the port. Boundary layer separation occurs when there is excessive expansion along the longitudinal axis of the port. The fluid expansion causes excessive momentum loss near the wall or contour of the port such that the flow breaks off or separates from the wall of the port.
To minimize boundary layer separation, many port designs use flares in the shape of a nozzle at opposing ends of the port to provide smooth transitions. Often, different flares are tried until the “best” one is found. In many flare designs, the performance of the port may be poor because boundary layer separation will occur at the point along the longitudinal axis of the port where the adverse pressure gradient is largest. The pressure gradient or change in pressure may become great enough that the momentum of the sound wave or fluid is greater than the pressure holding the sound wave to the wall or contour. In this case, the sound wave separates from the wall, thus generating noise and losses. The point where the maximum pressure gradient occurs along the port limits the flow velocity from the port before separation occurs. Once the sound wave or flow separates from the port contour or wall at the point of maximum pressure gradient, flow losses increase dramatically and result in poor performance of the port.